Organization of a reliable system of protective electrical safety is one of the main conditions that prevent harm to users of electrical installations. It will provide protection not only to humans, but also to devices. Properly designed and installed protective grounding will prevent the negative impact of a wide range of unpredictably occurring circulating currents, eliminate their short circuit on the cases. As a result, the probability of traumatic injuries and the failure of complex technical devices will be excluded.
- Protective earthing system action
- How are the calculations made?
- Classification of grounding devices
- Parameter Calculation Method
The purpose of protective grounding is to create electrical connections to earth of non-conductive metal elements that are at risk of voltage. The cause of the undesirable occurrence of voltage can be lightning discharges, short circuit to the housing, carrying out potential, induction that appears under the influence of nearby current-carrying devices or their parts and a number of other situations. The connection can be made with soil or its equivalent, such as sea or river water, coal in the quarry, other natural or artificially created objects with similar properties.
Protective earthing system action
The work of the protective grounding system is to reduce the parameters of step voltage and touch, in bringing them to safe values. As a result of a competent device of the grounding system:
- the potential of grounded electrical equipment is reduced;
- the parameters of the potential of the base on which the users stand are aligned with the potential of the grounded technical installation.
Important. In the absence of grounding of an electrical installation, touching its housing is as dangerous as touching a phase wire of an electrical network.
Based on the reduction of values or on the equalization of potentials of the grounded electrical equipment, the principle of protective grounding helps to reduce the voltage of the housing relative to the object used for grounding, which is often used as soil. Due to this, the current passing through the user’s body and the voltage of touch (step) will reach values that are absolutely safe for humans and technology.
The grounding function will be fully implemented if the earth fault current indicators do not increase due to a decrease in the grounding resistance. This condition is fully consistent with an isolated neutral network – with a generator or transformer device not connected to the grounding system or connected to it through a high resistance of various measuring, signaling, protective devices.
How are the calculations made?
Fundamentally, the calculation of protective grounding consists in accurately determining the main parameters. They are required to create a circuit that forms the most safe step and touch voltages that appear at the moment of phase closure to the housing. Based on the calculated values that are within the permissible limits, the number and dimensions of grounding conductors are calculated, the order of placement of single elements is planned.
Classification of grounding devices
By origin, grounding conductors are divided into two categories, when making calculations, it is necessary to take into account their specific differences and features:
- Natural objects represented by third-party conductive parts in direct contact with the ground. The category of natural grounding conductors also includes objects whose electrical contact with the earth is made through an intermediate conductive medium.
In a natural grounding conductor, soil is installed (hammered, lowered into drilled wells) vertical elements interconnected by horizontal
In addition to soil and water, the category of natural grounding conductors includes metal pipes for water and other utilities laid in a trench way. Pipelines with combustible and explosive substances, highways constructed with the partial use of PVC pipes and parts cannot be used as grounding conductors of natural origin. Designed to provide trouble-free functionality of electrical equipment in emergency and normal conditions, working and protective grounding, eliminating the likelihood of damage, are mainly installed in the ground.
- Artificial ground electrodes, most often represented by vertical or horizontal electrodes.
Parameter Calculation Method
To perform the calculations, the following data is required:
- characteristics of specific electrical equipment, such as the type of installation and its main devices, operating voltages, possible methods for grounding the neutrals of transforming and generating devices;
- the size and configuration of the electrodes, making it possible to take into account the estimated depth of their immersion in the ground;
- information on measurements of the resistivity of the soil layer in the territory equipped with the grounding system, the characteristics of a specific climatic zone (you can get them in the local weather service);
Protective grounding circuit shown in section
Important. When placing a protective grounding system in two soil layers, data on the resistivity of each of them is needed, accurate data on the power of the upper layer will be needed.
- information on the availability of suitable natural grounding conductors, on which objects can be used for grounding, will also require real indicators of the resistance to current spreading of these objects obtained by measurement;
- accurate indicators of the calculated earth fault current calculated by the standard method;
- the calculated characteristics of the permissible voltage norms and rules of the PES, the period of the grounding protection, which is necessary if the calculations were made according to the values of the touch voltage and the values of the step voltage.
Mainly protective grounding and zeroing of electrical installations are calculated for cases of installation of elements of the grounding system in a homogeneous soil. However, calculation methods have now been developed and are being applied with the arrangement of grounding conductors in a heterogeneous earth.
- The calculation of grounding conductors located in a homogeneous environment requires taking into account the values of the resistance of the seasonally freezing layer during periods of freezing and drying of the earth. To obtain accurate values, special coefficients used in calculations for grounding systems any complexity.
- The calculation of grounding conductors installed in two or more layers of soil requires taking into account the resistance values of all layers. The calculation is based on taking into account all the potentials induced on the installed electrodes that are part of a complex structure from the group of earthing switches.
Independent of the calculation method, a parameter common to all schemes is the required resistance, determined in accordance with the regulations of PES.
For electrical equipment with voltage up to 1 kV, the calculation of the resistance of the grounding element included in the protective grounding system with insulated neutral (type IT) is performed in accordance with the condition:
In this inequality, the variable Rz is the resistance value of the grounding device (expressed in Ohms), a constant value is Upr.add. – contact voltage parameter (50 V), Iz – total value of earth fault, expressed in A.
According to regulatory requirements, the value of Rz varies from 4 ohms to 10 ohms (there are no special requirements for the lower limit value, the upper one is the maximum permissible parameter), provided that the power of the mains supply and transformers and generators connected in parallel is not higher than 100 kVA.
To organize protective grounding of installations with a higher voltage, other values are used in the calculations:
- 0.5 Ohm in power grids with an effectively grounded neutral with their characteristic large currents to earth;
- not higher than 10 Ohms at 250V contact voltage in systems with isolated neutral (the condition is valid at low currents to the grounding object for equipment with a voltage of more than 1000 V).
Note. Minimum resistance values for ground loops are preferred. The lower its value, the more effective will be the movement of current through objects with the least resistance.
The current spreading resistance calculated for the installation of the grounding system, calculated for the grounding conductor, may increase during operation. Its value must be constantly monitored..
Circuit diagram and installation
In multi-apartment buildings, circuit breakers are installed to protect residents from damage;.
In autonomous housing and in summer cottages, due to the lack of technical ability to install disconnecting automation, a protective grounding device is required, which can be done by contacting electricians, or made with your own hands.
Grounding system without deep grounding
Elements of the circuit, which does not include a deep ground electrode, can be installed in a row or arranged in the form of some kind of geometric figure. The shape of the contour depends on the features of the site. This method is applicable when the length of the ground lines up to 3 meters.
- Vertical grounding penetrates. The distance between earthing switches installed vertically in the ground is calculated based on the length of these parts of the grounding system. This is necessary to minimize shielding, since the closer the elements are located, the greater the shielding effect.
- A step-by-step measurement of the real resistance values of individual grounding conductors is carried out. They must be installed in an amount that ensures the formation of minimum resistance..
Note. Often, a different number of grounding elements is used to ground equal areas of land, since the resistivity of the soil is influenced by the specific resistance of the soil.
- The connection of single earthing switches is in progress. Earthing switches having a corrosion-resistant coating are connected using special connectors. Grounding devices made of ferrous metal are connected exclusively by welding, the seams are covered with bitumen mastic.
Group system with artificial earthing
This is a simple, protective grounding circuit available for independent implementation with an isosceles triangle contour. This electrical protection system is located a meter from the lower border of the base or wall.
- In accordance with the selected configuration, a trench 0.8 meters deep is dug. The length of each side of the triangle is 3 m.
- At the vertices of the triangle, it is desirable to drill wells of three meter depth. If it was decided to hammer vertical grounding conductors with a sledgehammer, 1.5 m wells would be sufficient.
Note. It is much easier to hammer in objects with pointed ends. Sharpen the material you are going to hammer with a grinder.
As a material for vertical grounding, a metal pipe with a diameter of 50 mm, a steel bar of 10 mm, a corner with a side of 50 mm are suitable. It will take three segments three meters long. The horizontal grounding element can be made of nine meters of steel strip 4 mm thick, 40 mm wide.
- Then, the mounted ground loop is connected by welding to a conductor. It is made of round bar or strip steel. It connects to a natural grounding conductor..
The installed circuit will successfully perform the assignment of protective resistance if it is connected to a steel water pipe laid in the ground, to metal casing pipes of water wells, to other reinforced concrete and metal structures. After installing protective grounding, all trenches and excavations must be filled with homogeneous soil without the inclusion of crushed stone and construction debris.
Having got an idea of what is called protective grounding, having learned about the ways of constructing an electrical protective system, you can safely proceed with self-assembly. Do not forget about measuring the resistance value after installation. If there are no corresponding devices and skills, it is required to call electricians.